The invention relates to a multi-stage membrane suction pump having at least two pump chambers, each comprising one fluid inlet, provided with at least one inlet valve, and one fluid outlet, comprising at least one outlet valve, as well as one suction line connecting the fluid inlets of the pump chambers, with successive pump chambers each being connected to one another via at least one connection line such that the membrane pump, upon reaching/exceeding a differential pressure in the suction line, changing from a parallel operation of its pump chambers to at least also a serial operation of said pump chambers, with a check valve being interposed in the inlet and the outlet area of at least one connecting line, open towards the downstream pump stage, and with the check valves provided in the inlet area and in the outlet area of the connection line(s) being embodied smaller in reference to the inlet valves and the outlet valves of the pump chambers.
During evacuation, for example of an autoclave, on the one hand a great flow rate is desired and on the other hand a good final vacuum. The great flow rate is achieved by switching the heads to a parallel configuration, the good final vacuum by a multi-stage operation, i.e. by way of switching it to a serial configuration. In many applications, primarily in lab operations, a low final pressure is required, which can only be achieved by a multi-stage arrangement.
From WO 2004/088138 a micro-vacuum pump is already known comprising two pump chambers, each limited by an oscillating pump membrane. Each of these pump chambers has a fluid inlet, comprising an inlet valve, and a fluid outlet, comprising an outlet valve, with a suction line connecting the fluid inlets of the pump chambers and a pressure line connecting the fluid outlets being provided. The pump chambers are connected to each other via a connection line such that the micro-vacuum pump of the prior art, upon reaching and exceeding a predetermined differential pressure in the suction line, changes from a parallel operation of its pump chambers into a serial operation of said pump chambers. One check valve each is interposed, open to the pump stage downstream, both in the inlet area as well as in the outlet area of the connection line. In order to reduce the expenses connected to the production of the membrane suction pump of prior art the check valves interposed in the connection lines show a size similar to the inlet valves and outlet valves of the two pump chambers. Accordingly, the line section of the connection line provided between one of the check valves, on the one side, and the adjacent pump chamber, on the other side, is sized comparatively large. In order to allow in the starting phase of a pumping process to first guide the fluid flow via the inlet valves and outlet valves switched parallel the connection line is provided with an interposed throttle, which only loses its throttling effect when reaching a respective differential pressure and a reduced pump capacity.
At the beginning of the suction process the micro-vacuum pump of the prior art assumes a parallel operating configuration of its pump chambers, because the throttle provided in the connection line causes that the system, due to the still missing strain in the air circulation, can initially function easier in parallel operation. As soon as this parallel operating configuration reaches the range of the final vacuum and the differential pressure in the suction line thus reaches its maximum the fluid can flow much easier through the throttle located in the connection line so that simultaneously also a serial operation of its pump chambers is being configured in order to yield a final vacuum as high as possible.
However, it is disadvantageous that the check valves of the membrane pump of prior art show a size similar to that of the inlet valves and the outlet valves, and that the line sections of the connection line provided between the check valves have a line cross-section of a size according to the connection line such that in these line sections a correspondingly large, dead space develops, which has unfavorable effects upon the achievable final vacuum of the above-mentioned membrane suction pump and negatively influences the switchover point from parallel to serial operation.
A superheated steam vacuum pump having at least two stages is known from DE 10 2006 043 159 B3, comprising membranes operating in a push-pull mode as the pump organs. The inlets and the outlets of the pump chambers of this multi-stage superheated steam vacuum pump are connected to each other in a parallel fashion via pipelines. Furthermore, a control line is provided, connecting the pump chambers, comprising a check valve arrangement having one control valve each arranged at the beginning and the end of the control line. At the beginning of each pumping process, i.e. with only little differential pressure between the pump inlet and the pump outlet, the two stages of the pump operate parallel because the control line is blocked by the vacuum-control valves. Beginning at a certain differential pressure the vacuum control valves open and the two pumps essentially operate serially. In order to create a greater differential pressure between the pump inlet and the pump outlet, the valve organs of the control valves provided in the control line show a weight that is at least 30% lower than the valve organs of the check valves provided in the pump inlet and in the pump outlet.
A valve-controlled operation control for multi-stage gas conveyer pumps is already known from DE 202 02 190 U1. In the operation control of prior art, which comprises a combination of two check valves as well as one pressure relief valve and/or a membrane-controlled vacuum valve, in which an automatic switching of the type of operation is achieved such that, upon reaching a defined pressure and/or vacuum, the pressure relief valve and/or membrane-controlled vacuum valve open, the check valves close, and the flow is conveyed from the pressure side of one pump stage to the suction side of another pump stage. This way, the pump stages are switched from a parallel operation to a serial one.
In DE 10 2006 043 B3 and DE 202 02 190 U1 improved solution are described in detail only for a preferably two-stage superheated steam vacuum pump and/or a valve-controlled mode of operation.